1. Field of the Invention
The present invention relates to a semiconductor device.
2. Background of the Related Art
Enhancement of a protection function provided for preventing a semiconductor device per se from being broken down, enhancement of a current detection (current sense) function of detecting a current flowing into the semiconductor device, etc. as well as the increase of the current and the reduction of ON-resistance (low ON-state voltage) have been requested in the semiconductor device in the background art. A current sense semiconductor element (hereinafter referred to as current sense element) is an element for detecting a current flowing into a semiconductor substrate (semiconductor chip). Due to characteristics of the current sense element, the current sense element is therefore disposed on the same semiconductor substrate as a main element arranged in an active region is disposed. The current sense element has a similar cell structure to cells (element's functional units) constituting the main element. The active region is a region in which a main current flows in an ON time.
An area occupied by a region where cells constituting the current sense element are disposed (hereinafter referred to as current sense region) is determined based on a current sense ratio and reduced to be smaller than an area occupied by the active region. The current sense ratio is a conversion ratio for calculating a current actually flowing into the active region based on a current detected by the current sense element. As described above, the cells constituting the current sense element (hereinafter referred to as current sense cells) and disposed in the current sense region have the same structure as the cells (hereinafter referred to as active cells) constituting the main element and disposed in the active region. Therefore, the number of the current sense cells disposed in the current sense region is determined based on the current sense ratio.
As to the structure of the background-art semiconductor device having the current sense element which is provided on the same semiconductor substrate as the main element is disposed, an MOSFET (Metal Oxide Semiconductor Field Effect Transistor) will be described by way of example. FIG. 10 is a plan view showing a planar layout of the background-art semiconductor device. FIG. 11 is a sectional view showing a sectional structure in a cutting plane line AA-AA′ of FIG. 10. A planar layout of an active region 121, a current sense region 122 and respective electrode pads is shown in FIG. 10 (the same rule is also applied to FIG. 12). The sectional structure in the cutting plane line AA-AA′ passing through the active region 121 and the current sense region 122 is shown in FIG. 11.
As shown in FIG. 10, the background-art semiconductor device (hereinafter referred to as Background-Art Example 1) has the active region 121, the current sense region 122 and a termination structure portion 123 which are provided on the same semiconductor substrate. The active region 121 surrounds the current sense region 122. The termination structure portion 123 surrounds the active region 121. A boundary between the active region 121 and the current sense region 122 and a boundary between the active region 121 and the termination structure portion 123 are not shown. In the active region 121, a source electrode pad 111, a current sense electrode pad 112, and a gate electrode pad 113 are provided separately from one another on a front surface of the semiconductor substrate.
In the current sense region 122, a current sense electrode 110 is provided on the front surface of the semiconductor substrate to cover substantially the whole surface of the current sense region 122. The current sense electrode 110 is disposed between the source electrode pad 111 and the current sense electrode pad 112. The current sense electrode 110 is a front surface electrode shared among all current sense cells. The source electrode pad 111 is provided in substantially the whole surface of the active region 121. The source electrode pad 111 is opposed to a part of the current sense electrode 110, a part of the current sense electrode pad 112 and a part of the gate electrode pad 113. The source electrode pad 111 is a front surface electrode (source electrode) shared among all active cells.
The current sense electrode pad 112 and the gate electrode pad 113 are disposed in the active region 121 so as to extend near the boundary with the termination structure portion 123 and along an inner circumference of the termination structure portion 123. The current sense electrode pad 112 is disposed more closely to the outer circumferential portion of the chip than the current sense electrode 110, and opposed to the current sense electrode 110. The current sense electrode 110 is electrically connected to the current sense electrode pad 112. Gate electrodes of the respective active cells are connected to the gate electrode pad 113 through not-shown gate runners. The termination structure portion 123 is a region in which an electric field on the front surface side of an n− type drift layer made of the semiconductor substrate is relaxed to retain a breakdown voltage.
As shown in FIG. 11, on the front surface side of the semiconductor substrate serving as the n− type drift layer 101, a plurality of cells having the same cell structure are provided to extend from the active region 121 to the current sense region 122. That is, the current sense cells having the same cell structure as the active cells disposed in the active region 121 are provided in the current sense region 122 continuously to the active cells. Each of these cells is provided with a general trench gate structure. The trench gate structure includes a p-type base region 102, a trench 103, a gate insulating film 104, a gate electrode 105, an n+ type source region 106 and a p+ type contact region 107. Inside the trench 103, the gate insulating film 104 is disposed along an inner wall of the trench 103 and the gate electrode 105 is embedded.
A front surface electrode (source electrode) also serving as the source electrode pad 111 makes contact with the n+ type source regions 106 and the p+ type contact regions 107 of the active cells. The current sense electrode 110 serving as a front surface electrode makes contact with the n+ type source regions 106 and the p+ type contact regions 107 of the current sense cells. The current sense electrode 110 and the source electrode pad 111 are electrically insulated from the gate electrodes 105 by an interlayer insulating film 109. The symbol 108 designates a high temperature oxide (HTO) film. An n+ type drain layer and a drain electrode which are not shown are provided on a back surface side of the semiconductor substrate. The symbol w101 designates a width of a mesa region between adjacent trenches 103. The symbol w102 is a trench pitch.
The structure of another example of the background-art semiconductor device (hereinafter referred to as Background-Art Example 2) having a current sense element which is provided on the same semiconductor substrate as a main element is disposed will be described. FIG. 12 is a plan view showing a planar layout of the other example of the background-art semiconductor device. FIGS. 13A and 13B are sectional views showing sectional structures in a cutting plane line BB-BB′ and a cutting plane line CC-CC′ of FIG. 12 respectively. The sectional structure in the cutting plane line BB-BB′ is shown in FIG. 13A. The sectional structure in the cutting plane line CC-CC′ is shown in FIG. 13B. Background-Art Example 2 is different from Background-Art Example 1 (see FIG. 10 and FIG. 11) in the point that a current sense region 124 is disposed immediately under (on a drain side of) a current sense electrode pad 114. That is, the current sense electrode pad 114 also serves as a current sense electrode on a front surface of a semiconductor substrate.
Generally, the current sense region 124 has a structure to be surrounded by a diode region 125. That is, the diode region 125 is disposed between an active region 121 and the current sense region 124. A boundary between the active region 121 and the diode region 125 is not shown. The current sense electrode pad 114 is provided to extend from the current sense region 124 to the diode region 125 so that the current sense electrode pad 114 can not only entirely cover the substrate front surface in the current sense region 124 but also cover the substrate front surface in the diode region 125. Diode cells are disposed in the diode region 125. Each of the diode cells has a p-type base region 102 as an anode, and an n− type drift layer 101 and an n+ type drain layer (not shown) as a cathode. The current sense electrode pad 114 also serves as an anode electrode. A drain electrode (not shown) also serves as a cathode electrode.
In the Background-Art Example 1 (see FIG. 10 and FIG. 11), a part of the cells of the active region 121 are used as current sense cells of the current sense region 122. Therefore, the source electrode pad 111 and the current sense electrode 110 have to be separated from each other. Accordingly, it is difficult to separate the electrodes in the recent microminiaturized cells. In addition, it is also necessary to reduce the thickness of each electrode in order to microminiaturize the cells. However, when the thickness of the electrode within the active region 121 is reduced, ON-resistance increases or reliability during fabrication deteriorates. Accordingly, the Background-Art Example 1 can be applied only to a cell structure large in cell pitch. In order to apply the Background-Art Example 1 to a microminiaturized cell structure, it is necessary to add a multistage metal step to form the cell structure into a multistage metal structure. The multistage metal step means a step of laminating a plurality of metal films of different metal materials, thereby resulting in the increase of the number of steps. Further, in the Background-Art Example 1, the current sense cells are disposed continuously to the active cells. Therefore, the Background-Art Example 1 has a problem that a current flows into the current sense cells from the active region 121 to cause deterioration of current sense accuracy.
In the Background-Art Example 2 (see FIG. 12 and FIGS. 13A and 13B), the diode region 125 is disposed between the active region 121 and the current sense region 124. Accordingly, it is possible to suppress a current from flowing into the current sense region 125 from the active region 121. However, current density in an ON time is usually increased in a trench gate type MOS semiconductor device. Therefore, in order to improve ruggedness to avalanche breakdown, surge, etc., the breakdown voltage of the active region 121 having a large occupation area relative to the surface area of the semiconductor substrate is lowered and a current is shared equally among the active cells. Since each current sense cell has the same cell structure as each active cell, the breakdown voltage of the current sense region 124 becomes as low as the breakdown voltage of the active region 121.
In addition, since the active cell and the current sense cell have the same cell structure, avalanche breakdown occurs in the active region 121 and the current sense region 124 simultaneously. On this occasion, since a range including the current sense region 124 and a part of the diode region 125 is covered with the current sense electrode pad 114, avalanche breakdown occurs in pn junction faces between the p-type base regions 102 and the n− type drift layer 101 in the current sense region 124 and the diode region 125. Therefore, a current increasing suddenly due to the avalanche breakdown (hereinafter referred to as avalanche current) flows into the current sense region 124 not only from the current sense region 124 but also from the diode region 125.
Normally, in comparison with the surface area of the semiconductor substrate, the area occupied by the current sense region 124 is small but the area occupied by the diode region 125 surrounding the current sense region 124 is large. Therefore, a larger avalanche current flows into the current sense cells disposed in the current sense region 124 than into the ordinary active cells correspondingly to the avalanche current flowing from the diode region 125. Thus, ruggedness to avalanche breakdown of the current sense region 124 becomes lower than ruggedness of the active region. Accordingly, it is necessary to protect the current sense element from avalanche breakdown, surge, etc. in order to suppress the ruggedness of the current sense region 124 from decreasing.
Generally in the background art, a resistor is connected to the current sense element (hereinafter referred to as first background-art structure) or a protection circuit such as a resistor, a Zener diode, etc. is provided in a control circuit (not shown) connected to the current sense element (hereinafter referred to as second background-art structure), in order to suppress the ruggedness of the current sense region from decreasing. In the first background-art structure, a current generated due to surge etc. is suppressed from flowing into the current sense element. In the second background-art structure, since the control circuit is protected from surge etc., ruggedness of the current sense element is improved. However, as protection applied to the current sense element is reinforced more greatly, it is more difficult for a current to flow into the current sense element, resulting in the decrease of the current flowing into the current sense element or the increase of the current flowing into the current sense element due to a parasitic effect of the element or the circuit. Therefore, there is a fear that current sense accuracy may deteriorate.
The following device has been proposed as a device in which current sense accuracy is improved. The device is a trench gate type semiconductor device in which all detector cells, active cells and inactive cells are provided with dummy gate electrodes. A dummy gate electrode is disposed on the bottom of each trench through an insulating film and a gate electrode is formed on the dummy gate electrode through an insulating film. The depth of the trench is increased correspondingly to the dummy gate electrode provided on the bottom side of the trench (e.g. see JP-A-2009-182113, paragraphs [0102] and [0103], and FIG. 19)). In JP-A-2009-182113, the dummy gate electrode is provided or the trench is formed deeply. In this manner, a current flowing into each current sense cell (detector cell) from a region surrounding the current sense cell is suppressed and fluctuation of a current sense ratio is suppressed so that current sense accuracy can be improved.
The following device has been proposed as a device in which a current sense element is prevented from being broken down. A current detecting resistor is connected between a source electrode of a main element and a current sensing electrode of a current detecting element. A dielectric breakdown voltage of a gate insulating film is larger than the product of a maximum current which can flow into the current detecting element when a reverse bias voltage is applied and resistance of the resistor (e.g. see JP-A-2012-253391, paragraphs [0047] and [0048], and FIGS. 3 and 4)). In JP-A-2012-253391, a trench of each active cell (main element) is made deeper than a trench of each current sense cell (current detecting element) or a pitch between adjacent ones of the trenches of the active cells is made wider than a pitch between adjacent ones of the trenches of the current sense cells. Thus, the breakdown voltage of each current sense cell is made higher than the breakdown voltage of each active cell when a reverse bias voltage is applied.
However, in the first or second background-art structure, the number of steps increases because the protection unit against avalanche breakdown, surge, etc. is provided. Cost increases because it is necessary to secure an area for forming the protection unit on the same semiconductor substrate as the current sense element is disposed. In addition, in the first or second background-art structure, there is a fear that the current sense accuracy may deteriorate as protection applied to the current sense element is reinforced, as described above. In JP-A-2009-182113, since the dummy gate electrodes are provided or the trenches are formed deeply, the breakdown voltage of the current sense region decreases so that the ruggedness tends to decrease.
In order to solve the foregoing problems inherent in the background-art techniques, an object of the invention is to provide a semiconductor device in which current sense accuracy can be maintained while ruggedness of a current sense region can be improved.